Add prominent Gmail build callouts to AGENTS.md

Call out two facts that already caused a confusing failure, before the
build phases begin:

1. Force IPv4 for every Gmail connection (IMAP and SMTP).
2. The source address header is X-Pm-Original-To; never use Delivered-To.

Additive only. No FR id, phase id, or other section is changed.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

AGENTS.md

@@ -5,6 +5,19 @@
 
 ---
 
+## Critical build callouts
+
+Read these two facts before you start a build phase. They already caused a
+confusing failure once.
+
+1. **Force IPv4 for every Gmail connection.** This applies to IMAP and SMTP.
+   See §9 and the infrastructure notes. The first IMAP attempt failed with a
+   socket EOF over IPv6 from `.13`.
+2. **The source address header is `X-Pm-Original-To`** (added by PurelyMail).
+   See FR-2 and §13.7. **Never use `Delivered-To`** — Gmail overwrites it.
+
+---
+
 ## 1. Purpose and outcome
 
 A web app that triages the family inbox. All mail lands in one Gmail account